Systems and methods described herein generally relate to the field of ultrasound imaging. More specifically, embodiments described below relate to methods and systems for measuring shear wave velocity in tissue.
Pathological conditions may result in soft tissue which is stiffer than would be present under physiological conditions. Physicians therefore use palpation to locate stiff tissue within a body and thereby identify pathological conditions. For example, breast cancers are known to be generally harder than healthy breast tissue and may be detected as a hard lump through palpation.
The propagation velocity of shear waves in tissue is related to the stiffness (Young's modulus) of tissue by the following equation,E=3ρ·c2  (1)where c is the propagation velocity of shear wave, E is Young's modulus, and ρ is the tissue density. Therefore, cancers or other pathological conditions may be detected in tissue by measuring the propagation velocity of shear waves passing through the tissue.
A shear wave may be created within tissue by applying a strong ultrasound pulse to the tissue. The ultrasound pulse may exhibit a high amplitude and a long duration (e.g., on the order of 100 microseconds). The ultrasound pulse generates an acoustic radiation force which pushes the tissue, thereby causing layers of tissue to slide along the direction of the ultrasound pulse. These sliding (shear) movements of tissue may be considered shear waves, which are of low frequencies (e.g., from 10 to 500 Hz) and may propagate in a direction perpendicular to the direction of the ultrasound pulse. The ultrasound pulse may propagate at a speed of 1540 m/s in tissue. However, the shear wave propagates much more slowly in tissue, approximately on the order of 1-10 m/s.
Since the tissue motion is generally in the axial direction (i.e., the ultrasound pulse direction) the shear waves may be detected using conventional ultrasound Doppler techniques. In this regard, the ultrasound Doppler technique is best suited to detect velocity in the axial direction. Alternately, shear waves may be detected by measuring a tissue displacement caused by the acoustic radiation force.
In order to accurately measure the propagation velocity of the shear wave, the shear wave needs to be tracked at a fast rate or a fast frame rate of several thousands frames per second. An image in a frame may consist of a few hundred ultrasound lines. A typical frame rate of regular ultrasound imaging is about 50 frames/s, which is too slow to track the shear wave propagation. Therefore, there exists a need to increase the frame rate while maintaining a good signal to noise ratio and good spatial resolution. Also, there exists a need to efficiently provide an indication of tissue stiffness.
Shear wave velocity imaging may exhibit limited spatial resolution. Lateral spatial resolution may be particularly limited because the measurement of shear wave velocity requires detection of shear wave at at least two lateral positions (i.e., at the positions of at least two ultrasound beams). On the other hand, tissue strain may be detected at using only one ultrasound beam. Therefore, tissue strain imaging may exhibit higher spatial resolution than shear wave velocity imaging. However, tissue strain imaging may yield only a qualitative measurement of tissue stiffness while shear wave velocity imaging provides a quantitative measure.